Photosensitisation Facilitates Cross-priming of Adjuvant-free Protein Vaccines and Stimulation of Tumour-suppressing CD8 T Cells

Cancer vaccines aim to induce CD8 T cells infiltrating the tumour. For protein-based vaccines, the main biological barrier to overcome is the default MHC class-II-pathway, with activation of CD4 T cells rather than CD8 T cells. The latter requires antigens to access the cytosol and MHC class I antigen presentation. We applied photosensitiser and light to trigger disruption of antigen-containing endosomes and thereby MHC class I cross-presentation of a model cancer vaccine. This "photochemical internalisation" resulted in activation, proliferation, and IFN-γ production of cytotoxic CD8 T cells, which suppressed tumour growth by infiltrating CD8 T cells and caspase-3-dependent apoptosis. The process was independent of MHC class II, MyD88, and TLR4 signalling, but dependent on trypsin- and caspase-like proteasome activity and partly also on chloroquine. This novel method of vaccination may find applications in cancer immunotherapy where the activation of CD8 T cells is important.